In the practice of tennis, it is useful to have a means of viewing the change in the performance of a player over time, in particular with a portable display device, such as a mobile telephone, a tablet or a portable computer. Sensors installed in the racket make it possible to measure a certain number of parameters, in particular the number of strokes and the characteristics of these strokes, and to transfer them into a portable display device. However, the performance of a player can be represented in many ways and using multiples indexes, which makes clear and summarised viewing difficult.